Device and relative method for scavenging energy

ABSTRACT

An energy scavenging device includes an electromagnetic transducer adapted to generate a current in response to accelerations impressed thereto. The device also includes a power switching stage input with the current generated by the electromagnetic transducer, having a network of controlled switches adapted to alternately deliver on output nodes of the switching stage an output current that does not invert its sign and to short-circuit the transducer. There is an output capacitor coupled between the output nodes of the power stage. A controller having a sensor coupled to the electromagnetic transducer is to sense the current flowing therethrough, the controller being adapted to drive the switches of the power stage in order to either short-circuit the electromagnetic transducer or to direct the current flowing through the transducer to charge the output capacitor.

FIELD OF THE INVENTION

This invention relates to energy scavenging, and, more particularly, toa device for scavenging electric energy generated by electromagnetictransducers and related methods.

BACKGROUND OF THE INVENTION

Electromagnetic transducers convert motion or vibrations into energy.They generally include, as shown in the sectional view of FIG. 1, apermanent magnet adapted to slide into a solenoid. The equivalentcircuit of electromagnetic transducers is shown in FIG. 2.

Commonly, it is understood that the permanent magnet slides through thesolenoid with a sinusoidal oscillation motion, thus generating asinusoidal voltage V on the solenoid in an open circuit condition. Thevoltage generated by an electromagnetic transducer should be modeled asa sinusoidal voltage with random amplitude and frequency. For sake ofsimplicity, in the ensuing description it will be assumed that theelectromagnetic transducer generates a sinusoidal voltage with constantamplitude and frequency in open circuit conditions, though the sameobservations hold mutatis mutandis even if a sinusoidal voltage ofrandom amplitude and frequency is considered.

The electric energy generated by electromagnetic transducers isscavenged according to the electric scheme of FIG. 3, using a rectifyingbridge of the generated voltage and a common step-up converter chargingan output capacitor Cout. This approach is widely used, but itsefficiency is relatively poor.

SUMMARY OF THE INVENTION

Deep investigations carried out by the present inventors lead to aninference that, in prior energy scavenging devices, efficiency is poorbecause most of the electric energy generated by the electromagnetictransducer is dissipated by switching the five diodes and the switch ofthe step-up. Moreover, the step-up inductor is crossed by the whole DCcurrent that charges the output capacitor and this causes relevant Ohmiclosses, thus reducing the overall yield of prior energy scavengingdevices.

In order to address these issues, an energy scavenging device isproposed, containing an electromagnetic transducer and adapted tostep-up the voltage generated thereby, that may be realized using apower stage with a reduced number of active components (diodes+switches)and using the same inductance of the solenoid of the transducer as astep-up inductor, thus further reducing power losses of the device.

The switches of the power stage are driven by a controller having asensor coupled to the electromagnetic transducer to sense the currentflowing therethrough, the controller being adapted to drive the switchesof the power stage in order to either short-circuit the electromagnetictransducer or to direct the current flowing through the transducer tocharge the output capacitor.

The controller may implement various energy scavenging methods bydriving the switches of the power stage according to different controlrules. The invention is defined in the annexed claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electromagnetic transducer, according to the priorart.

FIG. 2 shows the equivalent circuit of an electromagnetic transducer,according to the prior art.

FIG. 3 shows a classic AC-DC step-up converter used for charging acapacitor Cout with an electromagnetic transducer, according to theprior art.

FIG. 4 depicts an architecture of an energy scavenging device, accordingto the present invention.

FIG. 5 depicts the equivalent circuit of the novel energy scavengingdevice of FIG. 4.

FIG. 6 is a flow chart of the functioning phases of the device of FIG.4.

FIG. 7 depicts the equivalent circuits of the device of FIG. 4 duringeach of the functioning phases of FIG. 6.

FIGS. 8A and 8B are time graphs that illustrate an embodiment of amethod of scavenging energy with the device of FIG. 4.

FIGS. 9A and 9B are time graphs that illustrate another embodiment of amethod of scavenging energy with the device of FIG. 4.

FIGS. 10A and 10B are time graphs that illustrate yet another embodimentof a method of scavenging energy with the device of FIG. 4.

FIGS. 11A and 11B are time graphs that illustrate yet another embodimentof a method of scavenging energy with the device of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of an architecture of an energy scavenging devicecontaining an electromagnetic transducer and adapted to implement astep-up rectification is depicted in FIG. 4 and its equivalent circuitis shown in FIG. 5. The device has two diodes and two switches, thusswitching losses are reduced over prior art devices. Moreover, Ohmiclosses are reduced by using the inductance of the solenoid of thetransducer also as a step-up inductor.

The device has a controller STATE MACHINE adapted to monitor the currentflowing throughout the transducer T and to close the high-side and thelow-side switches such to short-circuit the transducer T. The controllerSTATE MACHINE is then to configure the switches either to charge anoutput capacitor Cout, or to keep shorted the transducer until a desiredpre-established condition on the current i_(T) is met. The basicswitching phases are summarized in the flow-chart of FIG. 6 and in FIG.7.

During phase 1, the transducer T is shorted, then a switch is opened(phase 2) and the parasitic capacitance Cpar of a diode is charged. Whenthe capacitance Cpar is charged, the diode enters in a conduction state(phase 3) and the output capacitor Cout is charged by the transduceruntil the current i_(T) decreases (phase 4), and both switches areclosed again (phase 1).

The controller STATE MACHINE is adapted to control the switches such tokeep the transducer shorted when the current i_(T) is small. Therefore,the current i_(T) is stepped-up by oscillations of the transducer untilthis current is adapted to be rectified and used to charge the outputcapacitor Cout.

The controller STATE MACHINE may be a state machine, a purposelyprogrammed processor, or any other circuit adapted to control theswitches of the power stage for implementing any desired method ofscavenging energy.

The time graphs of FIG. B illustrate a first embodiment of a novelmethod of scavenging energy using the novel device of FIG. 4. Otherembodiments are illustrated by the time graphs of FIGS. 9, 10 and 11.

For ease of understanding, positive half-waves of the generated opencircuit voltage V are depicted, so the high-side switch is to be turnedon/off whilst the low-side switch is kept closed. Clearly, when positiveand negative half-waves are considered, the correct switch to be turnedon or off should be identified by checking the direction of the currenti_(T).

According to the embodiment of the method illustrated by the time graphsof FIG. 8, high-side and low-side switches are closed and the currenti_(T) flowing through the transducer T is sensed. When the current i_(T)attains a peak, the high-side switch is opened and the current generatedby the transducer charges the output capacitor Cout until it becomesnull (interval T1). Then the opened switch (in the shown example, thehigh-side switch) is closed again and the same procedure is repeated inthe next half-period. With this technique the energy transfer to theoutput capacitor is triggered twice in a period and when the timederivative of the current flowing through the transducer i_(T) becomesnull.

The above method may be conveniently used when the time interval T1during which energy is transferred to the output capacitor Cout is notmuch shorter than a quarter of a period of the voltage generated by thetransducer.

According to another embodiment of the method illustrated by the timegraphs of FIG. 9, which is particularly effective when the currentthrough the transducer rapidly drops to zero when charging the outputcapacitor Cout, the high-side (low-side for negative half-waves) switchis turned off/on a plurality of times with a certain fixed switchingfrequency during a same half-period of the voltage generated by thetransducer. The switching frequency should be determined such to allowthe current i_(T) to become equal to the ratio between the open circuitvoltage V of the transducer and the series resistance between theresistance Rtrans of the transducer and the on-resistances of theswitches.

With the above technique, energy is transferred to the output capacitorCout a plurality of times during a same half-period, that is more energymay potentially be transferred to the capacitor Cout, though switchinglosses are increased in respect to the previously described method.

According to yet another embodiment of the method illustrated by thetime graphs of FIG. 10, the transducer is kept shorted until the currentflowing therethrough crosses a certain nonnull current threshold i_(th),then the high-side (low-side for negative half-waves) switch is turnedoff in order to transfer energy to the output capacitor. When thecurrent i_(T) nullifies, the transducer is short-circuited again and thehigh-side (low-side) switch is turned off again when the current crossesagain the threshold i_(th), and so on.

With this technique, the turning on/off of the switches is triggeredwhen the current generated by the transducer attains the nonnullthreshold and it is not necessary to wait that the current i_(T) becomesproportional to the open circuit voltage V of the transducer. Accordingto a preferred option, the threshold i_(th) is determined such to makepositive the difference between the energy in the inductance of thetransducer and the energy lost during each switching by parasiticcapacitances of the device.

According to yet another embodiment of the method illustrated by thetime graphs of FIG. 11, the transducer is kept shorted until the currentflowing therethrough crosses the nonnull current threshold i_(th), thenthe high-side (low-side for negative half-waves) switch is turned off/ona plurality of times with a certain switching frequency during a samehalf-period of the voltage generated by the transducer. In practice,this technique is a combination of the two preceding techniques.

1-7. (canceled)
 8. An energy scavenging device, comprising: anelectromagnetic transducer configured to generate a current in responseto accelerations impressed thereto; a power switching stage to be inputwith current generated by said electromagnetic transducer, andcomprising a network of controlled switches and output nodes coupledthereto; said network of controlled switches configured to alternatelydeliver to said output nodes an output current that does not invert itssign, and to short-circuit said electromagnetic transducer; an outputcapacitor coupled between said output nodes; and a controller having asensor coupled to said electromagnetic transducer configured to sensecurrent flowing therethrough, said controller configured to drive saidnetwork of controlled switches to either short-circuit saidelectromagnetic transducer or direct the current flowing through saidelectromagnetic transducer to charge said output capacitor.
 9. Theenergy scavenging device of claim 8, wherein said output capacitor hasfirst and second terminals; and wherein said power switching stagecomprises: a high-side switch and a low-side switch coupled in series toshort-circuit said electromagnetic transducer when both closed, and eachhaving a common terminal coupled to said first terminal of said outputcapacitor; a high-side diode and a low-side diode each coupled betweensaid second terminal of the output capacitor and said common terminal ofsaid high-side switch and said low-side switch.
 10. An energy scavengingdevice, comprising: an electromagnetic transducer responsive toaccelerations; a power switching stage coupled to said electromagnetictransducer; an output capacitor coupled to said power switching stage;and a controller coupled to said electromagnetic transducer andconfigured to either short-circuit said electromagnetic transducer ordirect current flowing through said electromagnetic transducer to chargesaid output capacitor.
 11. The energy scavenging device of claim 10,wherein said controller is configured to direct the current flowingthrough said electromagnetic transducer to not invert its sign.
 12. Theenergy scavenging device of claim 10, wherein said output capacitor hasfirst and second terminals; and wherein said power switching stagecomprises: a high-side switch and a low-side switch coupled in series toshort-circuit said electromagnetic transducer when both closed, and eachhaving a common terminal coupled to said first terminal of said outputcapacitor; a high-side diode and a low-side diode each coupled betweensaid second terminal of the output capacitor and said common terminal ofsaid high-side switch and said low-side switch.
 13. A method ofscavenging energy using a device comprising an electromagnetictransducer configured to generate a current in response to accelerationsimpressed thereto, a power switching stage input with current generatedby the electromagnetic transducer, a network of controlled switchesconfigured to alternately deliver on output nodes of the power switchingstage an output current that does not invert its sign and toshort-circuit the electromagnetic transducer, an output capacitorcoupled between the output nodes, a controller having a sensor coupledto the electromagnetic transducer to sense current flowing therethrough,the controller being configured to drive the switches of the powerswitching stage, the method comprising: monitoring the current flowingthrough the electromagnetic transducer using the controller; and drivingthe switches of the power switching stage, using the controller, toshort-circuit the electromagnetic transducer, and depending on thecurrent, either continuing to short-circuit the electromagnetictransducer or directing the current flowing through the electromagnetictransducer to charge the output capacitor until the current through theelectromagnetic transducer nullifies.
 14. The method of claim 13,further comprising: comparing a time derivative of the current flowingthrough the electromagnetic transducer with a null value, using thecontroller; and configuring the switches to charge the output capacitorwhen the time derivative crosses the null value until the currentthrough the electromagnetic transducer nullifies, using the controller.15. The method of claim 13, further comprising configuring, with a fixedswitching frequency, the switches to charge the output capacitor untilthe current through the electromagnetic transducer nullifies, then toshort-circuit the electromagnetic transducer, using the controller. 16.The method of claim 12, further comprising: comparing the currentflowing through the electromagnetic transducer with a non-nullthreshold, using the controller; when the current flowing through theelectromagnetic transducer exceeds the nonnull threshold, configuringwith a fixed switching frequency the switches to charge the outputcapacitor until the current through the electromagnetic transducernullifies, then to short-circuit the electromagnetic transducer, usingthe controller.
 17. The method of claim 13, further comprising:comparing the current flowing through the electromagnetic transducerwith a nonnull threshold, using the controller; and keeping theelectromagnetic transducer short-circuited when the current is smallerthan the nonnull threshold, and directing the current flowing throughthe electromagnetic transducer to charge the output capacitor until thecurrent through the electromagnetic transducer nullifies, when thecurrent is larger than the nonnull threshold.
 18. A method of operatingan energy scavenging device, comprising: configuring an electromagnetictransducer to generate a current in response to accelerations; inputtinga power switching stage with current generated by the electromagnetictransducer; configuring a controller coupled to the electromagnetictransducer to either short-circuit the electromagnetic transducer ordirect current flowing through the electromagnetic transducer to chargean output capacitor coupled to the power switching stage.
 19. The methodof claim 18, wherein said controller is configured to direct the currentflowing through said electromagnetic transducer to not invert its sign.20. The method of claim 18, further comprising: comparing a timederivative of the current flowing through the electromagnetic transducerwith a null value, using the controller; and configuring the controllerto charge the output capacitor when the time derivative crosses the nullvalue until the current through the electromagnetic transducernullifies.
 21. The method of claim 18, further comprising configuring,with a fixed switching frequency, the controller to charge the outputcapacitor until the current through the electromagnetic transducernullifies, then to short-circuit the electromagnetic transducer.